1. Field of the Invention
This invention relates to an improved process for the production of a purified n-propanol by the hydrogenation of propionaldehyde and the fractional distillation of the resulting crude n-propanol.
2. Background Information Including a Description of Related Art
It is known to produce n-propanol by the hydrogenation of propionaldehyde obtained, for example, by the hydroformylation of ethylene by reaction with carbon monoxide and hydrogen. However, in order to be suitable for various applications, e.g., as a solvent for vegetable oils, waxes, resins, and cellulose esters and ethers in the manufacture of polishing compounds, brake fluids and various propyl compounds, the n-propanol must have a high degree of purity including a specified low level of certain impurities produced by the hydroformylation and hydrogenation reactions. To deal with this problem, the crude n-propanol produced by the hydrogenation reaction must be purified, generally by fractional distillation. When any of conventional catalysts such as Raney nickel is used for the hydrogenation of propionaldehyde to n-propanol, undesirably large amounts of certain impurities tend to be formed in the presence of little or no water as a result of side reactions of the hydrogenation reaction, with two of the more significant impurities being n-propyl propionate (prpr) and di-n-propylether (DPE). To reduce the amounts of such impurities formed as a result of these side reactions, the crude propionaldehyde feed to the hydrogenation reaction when Raney nickel is employed as the hydrogenation catalyst, is adjusted to contain a significant amount of water, e.g., at least about 4.0 wt % and up to about 10 wt %, based on the weight of the feed to the hydrogenation reaction, resulting in approximately the same percentage of water in the crude n-propanol product from the reaction, based on the weight of such product. However, while this amount of water results in decreased production of prpr and DPE by-products from the hydrogenation reaction, it renders the separation of prpr from n-propanol in the fractioning column more difficult despite the prpr boiling point of about 122.degree. C., since a prpr/water azeotrope has a boiling point of about 88.degree. C., giving rise to the presence of liquid prpr above the withdrawal point of purified n-propanol in the column. Moreover, the presence of such a large amount of water in the fractionating column makes the recovery of purified n-propanol from the fractionating column more difficult since n-propanol forms a binary azeotrope with water having a boiling point of about 87.degree. C., and a ternary azeotrope with water and DPE having a boiling a point of about 74.degree. C., which must be withdrawn from the column as streams separate from the stream of purified n-propanol having a boiling point of about 97.degree. C. and must be separately purified. Finally, the presence of a relatively large amount of water results in a substantial expenditure of energy, generally through steam consumption, to vaporize the water present, and may also necessitate a larger purification column than would otherwise be necessary to carry out the purification. In view of this, any change in the process is desirable which results in a decreased amount of water necessary in the reactor and the fractionating column and thus a reduction in energy consumption and possibly the size of the column, without any increase in the amount of prpr present in the product which would ordinarily result from the reduced presence of water in the hydrogenation reaction.
The following prior art references may be considered material to the claimed invention.
U.S. Pat. No. 4,263,449, issued Apr. 21, 1981 to Saito et al., discloses a process for producing alcohols, e.g., propanol, by the hydroformylation of an alkenyl compound, e.g., ethylene, and the hydrogenation of the resulting aldehyde in the presence of a hydrogenation catalyst, e.g., Raney cobalt. Water is added at a ratio of 0.5 to 30 times by weight based on the aldehyde produced by the hydroformylation before the hydrogenation.
U.S. Pat. No. 4,826,799, issued May 2, 1989 to Cheng et al., discloses a process of making catalysts by the Raney process including the steps of pelletizing a Raney process metal alloy, e.g., of cobalt and aluminum, in a matrix of polymer and plasticizer followed by removal of plasticizer or plasticizer and polymer, and the leaching out of the aluminum with caustic. The catalyst may be used to hydrogenate an aldehyde to the corresponding alkanol.